The present invention relates to a process for preparing selected o-nitroazobenzene compounds which are intermediates in the subsequent preparation of 2-aryl-2H-benzotriazole UV-absorbing stabilizers.
The instant process involves the coupling of a substituted phenol with diazotized o-nitroanilines in a strongly alkaline aqueous lower alkanol solution. It is advantageous to minimize the water content in the reaction mixture in order to facilitate solubility of the substituted phenols.
The instant process is distinguished from the prior art by the use of an excess amount of alkali metal hydroxide beyond the stoichiometric amount necessary to neutralize all the mineral acid added with the acidic diazonium salt solution and to convert the phenols into their alkali salts. Accordingly throughout the instant process coupling occurs in the presence of an excess of hydroxyl ion and at a pH value substantially over 11.
The success of the instant process in preparing the instant o-nitroazobenzene intermediates is surprising in view of a panoply of existing references which teach against such a high caustic coupling process.
While the coupling of phenols with diazonium salts under moderately alkaline conditions is a well known process for the synthesis of aromatic azo derivatives, it is pointed out by J. D. Roberts and M. C. Caserio, ("Basic Principles of Organic Chemistry," W. A. Benjamin, Inc., New York (1965), pages 893-895) that the coupling procedes at a maximum rate near pH of 10 and that at higher pH values the coupling virtually ceases.
Furthermore it is known that o-nitrobenzenediazonium salt solution is unstable in the presence of alcohols and alkali metal hydroxides. Rapid decomposition takes place with the evolution of elemental nitrogen and other side reactions. For this reason the prior art teaches that excess alkali should be avoided in coupling reactions. See Fierz-David et al, "Fundamental Processes of Dye Chemistry," Interscience, New York (1949), pages 239-241, 252-253.
H. Zollinger "Azo and Diazo Chemistry," Interscience, New York (1961), pages 249-250 notes that too alkaline a solution for coupling is not recommended because the diazo equilibrium fails to remain on the side of the diazonium ions.
Russian Pat. No. 360,357 discloses the coupling of a sodium salt of a phenol in a mildly alkaline aqueous solution by the addition of an acidic diazonium salt solution having excess acid so that the pH of the reaction mixture falls rapidly during the course of the reaction to an acidic pH value well below the pH value of over 11 required for the instant process.
All of these references teach away from the instant process having an excess hydroxyl ion content and a lower alkanol cosolvent which gives high yields of the o-nitroazobenzene compounds of high purity.